Ab initio resonant Raman spectra of diamond-like carbons.
Résumé
Raman spectroscopy is a standard tool for the characterisation of carbon materials, from graphite to diamond-like carbon and carbon nanotubes. An important factor is the dependence of the Raman spectra on excitation energy, which is due to resonant processes. Here, we calculate the resonant Raman spectra of tetrahedral amorphous carbon. This is done by a tight-binding method, using an approach different from Placzek's approximation, which allows calculation of Raman intensities also in resonant conditions. The calculated spectra confirm that the G peak arises from chains of sp2 bonded atoms and that it correlates with the atomic and electronic structure of the samples. The calculated dispersion of the G peak position with excitation energy follows the experimental observations. Our ab initio calculations also show that the sp3 phase can only be seen by using UV excitation above 4 eV, confirming the assignment of the T peak at not, vert, similar1060 cm−1, seen only in UV Raman measurements, to C–C sp3 vibrations.